Scientist hits back at dam claims
The Waimea River, near Nelson, will be dry most summers if more water is pumped from the aquifers under the plains without augmentation, according to Landcare Research water scientist Andrew Fenemor.
If minimum flows in the river were to be maintained and seawater intrusion avoided, there needed to be limits on water taken from the aquifers, he said.
Fenemor is a former Tasman District Council environmental manager and a member of the newly formed Community Water Solutions Advisory Group, set up to advise the council and its proposed joint-venture partner in the $82.5 million dam project, Waimea Irrigators Ltd.
The dam is earmarked for the Lee Valley and tipped to be funded by a mix of ratepayer, irrigator and Crown funding.
Fenemor’s comments come in response to claims from a recently formed incorporated society, Water Information Network Inc (WIN), that there is ‘‘abundant’’ capacity in the unconfined aquifer, from which most water is drawn for urban and rural use.
At a recent series of public meetings, WIN secretary, researcher and former Tasman District Council candidate Murray Dawson argued the aquifer had an ‘‘abundant safety margin’’ of ‘‘reserve water’’ down to sea level.
However, Fenemor said ‘‘reserve water’’ was not ‘‘available water’’.
‘‘If ‘reserve water’ is used from the existing network of wells across the plains to meet projected demand and security of supply needs, the Waimea River will be dry most summers,’’ he said. ‘‘Aquifer pumping has a big influence in reducing river flows.’’
The unconfined aquifer and the river were basically ‘‘one and the same’’, he said.
Under the augmentation proposal, water would be released from the dam on a day-by-day basis to maintain the minimum flow in the Waimea River and provide sufficient water to maintain high aquifer levels throughout summer to meet daily pumped water demands.
‘‘Water released from storage increases the water depth and width across the riverbeds and the slight increase in pressure between river and underlying aquifer means there is increased leakage to maintain water storage in the aquifers through summer,’’ Fenemor said.
At the meetings, Dawson also claimed saltwater intrusion was a ‘‘red herring’’ and a ‘‘non issue’’.
Salt levels had never been above the Ministry of Health guideline, Dawson said.
However, Fenemor said two bores on the council’s Waimea wellfield were shut down permanently after seawater was drawn into them during a drought in 2001 ‘‘when pumping intercepted the saltwater wedge in the lower Waimea River’’.
The saltwater trigger was an electrical conductivity trigger in the Tasman Resource Management Plan.
It was not related to the drinkability of the water, Fenemor said.
Dawson pointed out that irrigator pumping was not increasing and Fenemor agreed, adding that was expected ‘‘as there has been a moratorium on new water takes since about 1996’’.
A main aim of the dam project was to provide an adequate water supply for all likely urban and rural demands for the next 100 years, he said.
The debate over the hydrology behind the project has a long history. Fenemor was part of a 2016 peer review of the hydrological science underpinning the design and operation of the proposed dam that found it was ‘‘fit for purpose’’.